I use one on a carb engine. You calibrate them before use and at least once a year. I use it as a guide because I’m running twin carbs at 5k feet altitude.
I watch my plugs but it’s nice to have the afr. gauge when changing jets and other carb tuning.
I have one on my '68 F250 (390 FE motor with an 1850 Holley), but I too only use it as a "tuning tool", as I don't have a clue what's in that engine or what it needs.